The present invention relates generally to control apparatus, and particularly to control apparatus for a drying kiln.
Large enclosures are used as kilns for removing moisture from lumber products by circulation of heated air within the kiln. For example, green lumber is stacked for drying by placing stickers between each layer of lumber to permit airflow therethrough and the stacks are placed in a heated building structure, i.e., kiln, with controlled ventilation and circulation to pass sufficient air through the stacks and carry away the moisture of the lumber. In such control systems the kiln may include various sensors for detecting kiln conditions such as dry-bulb and wet-bulb sensors and mechanisms for introduction of new air, expulsion of moisture laden air, circulation of air and operation of a heating system for maintaining given conditions, e.g., dry-bulb and wet-bulb setpoints, within the kiln.
The process of drying lumber within a kiln is driven primarily by the current moisture content of the lumber within the kiln. A drying schedule determines the specific control steps taken, typically variation in kiln conditions such as a schedule of dry-bulb and wet-bulb setpoints as a function of current moisture content. The schedule advances generally as a function of the amount of moisture in the lumber, and not consistently as a function of time. The load moisture content of each load can be different and kiln external conditions, which affect drying time, vary as well. As the lumber becomes drier, the control process is modified according to a selected drying schedule in order to dry the wood in an energy efficient and timely manner.
Under current practice, an operator cuts sample boards and makes drying pockets in the packs or loads of lumber to be placed into the kiln for drying. This step is done while stacking the lumber, or just before the kiln is loaded. Normally there are six to eight sample boards per kiln charge. If the lumber is green and not air dried, the operator will start-up the kiln on the first step of a given drying schedule specifying, for example, particular dry-bulb and wet-bulb setpoints. While the kiln is running the operator will take each sample board and cut a wafer section from each end of each board. The operator will then weigh each wafer and write its weight on the wafer, and will weigh each sample board and write its weight on the sample board. After numbering for identification each wafer and each sample board, the operator will end-coat the sample boards and place the boards back into the pocket of the load in the dry kiln to continue drying of sample boards with the rest of the lumber in the kiln. The operator then places the wafers in the oven and dries them completely. This normally takes from 20 to 24 hours. Then, using a formula for figuring moisture content, the original moisture content of each wafer is calculated. Adding two moisture content values for each wafer and dividing by two the operator can calculate the moisture content for the sample board before it was placed into the kiln. Using the average moisture content of the wafers, the original weight of the sample boards, and the formula for ovendry weight, the ovendry weight for each sample board can be calculated. The operator then pulls the sample boards out of the kiln and writes the ovendry weight for each sample board on it for future reference, and then puts it back into the pocket in the kiln. The operator then knows the moisture content of each sample board and by taking the moisture content for the three wettest boards and averaging these moisture content values the moisture content for the load is obtained and suitable setpoints are derived as a function of that load moisture content from the selected drying schedule. More particularly, the kiln may be started at step 1 in the drying schedule, but with the ,current moisture, content it is possible to skip subsequent steps in the drying schedule, e.g., go directly from step 1 to step 3, based on the current moisture content of the load. For example, on green oak and the more difficult to dry hardwoods, the operator will probably pull the sample boards each day to check them for moisture content and in response to the calculated moisture content adjust the drying schedule appropriately.
As may be appreciated from the above description of conventional kiln operation, many mistakes can be made as many calculations are performed by the operator and many transfers of hand-written information are performed. Accordingly, with such complex operator interaction and required steps there is a corresponding greater opportunity for math errors and errors in transcribing data by hand. As a result, the conventional practice of kiln operation can offer opportunity for inappropriate drying of lumber, and for excess waste of energy resources in connection with the kiln operation.
Accordingly, it is desirable that kiln operation be made more automatic and more fully support the operator in avoiding such opportunities for error. Furthermore, it is desirable that the kiln operator be provided with greater information regarding anticipated timing for advancing the drying process to the next step in the drying schedule. Under conventional practice, the operator must check the sample boards as a routine procedure and may check those sample boards several times before the calculated load moisture content indicates a required change in the drying schedule, i.e., a change in established setpoints for the kiln. Also, under present practice the operator may miss a required change in the drying schedule and the kiln will for a time inappropriately operate, i.e., be energy inefficient, with respect to the actual moisture content of the load.
Under more automated conventional practice, some kiln control systems offer a computer system with a selection of drying schedules obtained by menu driven operation of the computer. The computer can automatically advance to a subsequent step in such drying schedules, but only in response to manual input of moisture content data, i.e., as derived by the operator according to the method described above and manually input into the computer as moisture content values. It would be desirable, therefore, that the operator not perform all the steps and procedures associated with obtaining such moisture content values, rather that the operator need only perform a minimally complex procedure for checking the current condition of a load within a kiln. Under conventional practice, in order to perform this step the operator must intermittently derive moisture content values for the sample boards. As described above, this procedure requires many steps and specific calculations to be performed by the operator.
It is desirable, therefore, that a kiln control scheme allow less sophistication or less expertise on the part of the operator and allow the overall process to be more efficiently implemented with less human interaction with respect to calculations and manipulation of sample boards.